The present invention relates to a method of manufacturing an electronically synchronizable indicator instrument having a pointer which is turned by a stepping motor operating via a multi-step gearing between a zero stop and a scale end, wherein the stepping motor has a two-pole rotor of magnetizable material, two coils arranged perpendicular thereto, and a housing surrounding the coils, the motor being controlled by a microprocessor.
In such indicating instruments, in view of the insertion of the multi-step gearing between motor pinion and pointer shaft no reliable conclusions as to the position of the pointer can be drawn from the number of steps stored in the counter of the microprocessor, i.e. from the position of the rotor of the stepping motor, since the numerical value stored may be faulty for various reasons. In order that no erroneous indications result therefrom, the counter must be constantly resynchronized with the position of the pointer For this in the method in accordance with U.S. Pat. No. 5,032,781, the zero position of the pointer defined by a mechanical stop is used in combination with a test program in which the stepping motor is advanced in the direction of the zero position in the sense of a movement of the pointer until the end of the movement of the pointer is noted from the absence of an induced voltage in that winding of the stepping motor which is not being passed through at the time by current for the driving of the pointer. The counter is then set to zero or to the value corresponding to the zero position of the pointer.
This method operates satisfactorily but in the case of 90.degree. steps per revolution of the rotor requires four switching and four test routines, which is too cumbersome from a switching standpoint. In German Patent Application P 42 00 551, corresponding to U.S. application Ser. No. 07/959,736 filed Oct. 13, 1992, it has therefore been proposed that a given orientation of the two-pole rotor be associated with the zero position of the pointer upon the assemblying of the stepping motor and the indicating instrument. At the start of the synchronization process, the rotor is, in each case, first brought into this given orientation. In order to note the induction or the absence thereof, it is then necessary also to test the position of the rotor which follows this given orientation by one step. By the associating of a given orientation of the rotor with the zero position of the pointer, a sort of one-time mechanical presynchronization is obtained. In this way, assurance is had that the zero position of the pointer can be reached only upon a given step of the rotor, as a result of which, in the event of 90.degree. steps, in each case only one of four steps must be checked and the expense for switching can be reduced by 50% as compared with the old method since now only one of the two windings need still be connected alternatively to the source of voltage and the measuring mechanism.
However, it has been found that the associating of the pointer position with the rotor position which is to be effected upon the assembly entails certain difficulties. In one concrete case of use, a three-step gearing with a transmission ratio of 1:43 is provided. In this case one must expect a gearing play of about 30.degree.. In order to avoid this, a spring is provided which urges the pointer in one direction of rotation. This urging is maintained also upon reversal of the direction of rotation.
In order to reduce the noise, only an odd number of gear wheels is used. This has the result that the gearing must be assembled in a very specific constellation if rotor and pointer are to be associated with each other in the manner necessary for the shortened process of synchronization. The assembling of the gearing is therefore very tedious and furthermore entails considerable possibilities of error.